Method to minimize compatibility error in hierarchical modulation using variable phase

ABSTRACT

The present invention provides a method, receiver and transmitter for use in a SDAR system. The method involves generating a first modulated signal based on first input data. Additional modulation is superimposed on the first modulated signal based on additional input data, being spread across a plurality of symbols in the first modulated signal in a predetermined pattern to generate a modified signal which is then transmitted. The modified signal is decoded by performing a first demodulation of the first modulated signal then additional demodulation is performed to obtain additional input data. The superimposing step uses a plurality of offset sequence values to add the additional modulation to the first modulated signal. The offset sequence may appear as a pseudo-random distribution of offset sequence values, and may include at least one zero offset value. Alternatively, the additional modulated signal may be a fromed as a direct sequence spread spectrum modulation and the offset sequence appearing as a pseudo-noise distribution. A Hadamard matrix sequence may be used as the direct sequence code.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/525,616 filed on Nov. 26, 2003.

TECHNICAL BACKGROUND

The present invention generally relates to the transmission of digitaldata, and more particularly, to the transmission of digital data in asatellite digital audio radio (“SDAR”) system.

BACKGROUND OF THE INVENTION

In October of 1997, the Federal Communications Commission (FCC) grantedtwo national satellite radio broadcast licenses. In doing so, the FCCallocated twenty-five (25) megahertz (MHz) of the electromagneticspectrum for satellite digital broadcasting, twelve and one-half (12.5)MHz of which are owned by XM Satellite Radio, Inc. of Washington, D.C.(XM), and 12.5 MHz of which are owned by Sirius Satellite Radio, Inc. ofNew York City, N.Y. (Sirius). Both companies provide subscription-baseddigital audio that is transmitted from communication satellites, and theservices provided by these and other SDAR companies are capable of beingtransmitted to both mobile and fixed receivers on the ground.

In the XM satellite system, two (2) communication satellites are presentin a geostationary orbit—one satellite is positioned at longitude 115degrees (west) and the other at longitude eighty-five (85) degrees(east). Accordingly, the satellites are always positioned above the samespot on the earth. In the Sirius satellite system, however, three (3)communication satellites are present that all travel on the same orbitalpath, spaced approximately eight (8) hours from each other.Consequently, two (2) of the three (3) satellites are “visible” toreceivers in the United States at all times. Since both satellitesystems have difficulty providing data to mobile receivers in urbancanyons and other high population density areas with limitedline-of-sight satellite coverage, both systems utilize terrestrialrepeaters as gap fillers to receive and re-broadcast the same data thatis transmitted in the respective satellite systems.

In order to improve satellite coverage reliability and performance, SDARsystems currently use three (3) techniques that represent differentkinds of redundancy known as diversity. The techniques include spatialdiversity, time diversity and frequency diversity. Spatial diversityrefers to the use of two (2) satellites transmitting near-identical datafrom two (2) widely-spaced locations. Time diversity is implemented byintroducing a time delay between otherwise identical data, and frequencydiversity includes the transmission of data in different frequencybands. SDAR systems may utilize one (1), two (2) or all of thetechniques.

The limited allocation of twenty-five (25) megahertz (MHz) of theelectromagnetic spectrum for satellite digital broadcasting has createda need in the art for an apparatus and method for increasing the amountof data that may be transmitted from the communication satellites to thereceivers in SDAR systems.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for increasingthe amount of digital data that may be transmitted from communicationsatellites to receivers in SDAR systems. In doing so, the presentinvention provides an advantage over the prior art. While hierarchicalmodulation schemes have been previously used in other data transmissionapplications (e.g., Digital Video-Broadcasting—Terrestrial [DVB-T] andDVB-Satellite [DVB-S] systems), until now, such hierarchical modulationschemes have not been envisioned for use in SDAR systems. By introducingthe use of hierarchical modulation in SDAR systems, the presentinvention increases the amount of data that may be transmitted in SDARsystems and enables the enhanced performance of the receivers thatreceive the satellite-transmitted signals in SDAR systems.

In one form of the present invention, a method for transmitting twolevels of data in a hierarchical transmission system achieves theseends. The method includes generating a first modulated signal based onfirst input data. One or more additional modulations are superimposed onthe first modulated signal based on additional input data, eachadditional input data being spread across a plurality of symbols in thefirst modulated signal as a predetermined pattern to generate a modifiedsignal which is transmitted. The modified signal is decoded byperforming a first demodulation of the first modulated signal to obtainthe first input data and then additional demodulations to obtain theadditional input data.

In another form of the present invention, a receiver is provided forreceiving two levels of data in a hierarchical transmission system. Thereceiver has an antenna for receiving RF signals with a demodulatorcoupled to the antenna for downconverting received RF signals. A firstdetector is coupled to the demodulator and has a first output andcapable of providing digital information based on a first level of dataon the first output. Additional detectors are coupled to the demodulatorand have additional outputs, which are capable of providing digitalinformation based on an additional level of data on the additionaloutputs. The additional detectors are adapted to detect said additionallevels of data as predetermined patterns spread over the first level ofdata.

In still another form, the present invention provides a transmitter fortransmitting two levels of data in a hierarchical transmission system.The transmitter has an encoder adapted to receive a first and secondinput data. The encoder is capable of providing digital informationbased on a first level of data on an output stream with a second levelof data being superimposed as predetermined patterns spread over thefirst level of data. A modulator is coupled to the encoder forupconverting said output stream, with an antenna coupled to themodulator for transmitting RF signals based on the upconverted outputstream.

In other forms of the present invention, a plurality of offset sequencevalues spread the second modulation across the first modulated signal.The offset sequence may be a pseudo-random distribution of offsetsequence values, and may include at least one zero offset value.Alternatively, the second modulated signal may be a combination ofdirect sequence spread spectrum modulations or combinations of aHadamard matrix sequence.

BRIEF DESCRIPTION OF THE DRAWING

The above-mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is an illustrative view of a constellation chart for64-quadrature amplitude modulation (QAM) with an embedded quadraturephase shift keying (QPSK) stream;

FIG. 2 is a diagrammatic view of a SDAR system implementing a method ofthe present invention;

FIG. 3 is a block diagram of a SDAR communication system adapted toenable a method of the present invention;

FIG. 4 is a diagrammatic view of a QPSK constellation;

FIG. 5 is a diagrammatic view of a binary phase shift keying (BPSK)constellation;

FIG. 6 is a diagrammatic view of a hierarchical 8-PSK constellation; and

FIG. 7 is a flow chart illustrating a method of the present invention asutilized in a SDAR receiver.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings representembodiments of the present invention, the drawings are not necessarilyto scale and certain features may be exaggerated in order to betterillustrate and explain the present invention. The exemplifications setout herein illustrate embodiments of the invention in several forms andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DESCRIPTION OF INVENTION

The embodiments disclosed below are not intended to be exhaustive orlimit the invention to the precise forms disclosed in the followingdetailed description. Rather, the embodiments are chosen and describedso that others skilled in the art may utilize their teachings.

For the purposes of the present invention, certain terms shall beinterpreted in accordance with the following definitions.

Baseband: A signal whose frequency content is in the vicinity of directcurrent (DC).

Carrier: A single frequency electromagnetic wave the modulations ofwhich are used as communications signals.

Channel: A propagation medium for communication such as a path alongwhich information in the form of an electrical signal passes (e.g.,wire, air, water).

Data rate: The amount of data, or number of symbols, which may betransmitted on a signal per a unit of time.

Detector: A circuit that is capable of determining the content of asignal.

Downconvert: To convert a radio frequency signal from a higher to alower frequency signal for processing (i.e., to baseband).

Downlink: To transmit data from a satellite to a receiver on earth.

Feed Forward Correction (FFC): A method of improving secondary datadetection. By knowing the relative “I” (in-phase) and “Q” (quadrature)components of a constellation quadrant, the secondary detector may beenhanced to perform better by having a priori knowledge from the firstdetector to assist detection.

First Level Data and/or Primary Data: Existing data that may beinterpreted by current (i.e., “legacy”) SDAR receivers. Because thefirst level data can be interpreted by the legacy receivers, the firstlevel data may also be considered to have backwards compatibility.

Hierarchical Modulation: A method in which two separate data or bitstreams are modulated onto a single data stream by superimposing anadditional data stream upon, mapped on, or embedded within the primarydata transmission. The additional data stream may have a different datarate than the primary data stream. As such, the primary data is moresusceptible to noise than it would be in a non-hierarchical modulationscheme. The usable data of the additional stream may be transmitted witha different level of error protection than the primary data stream.Broadcasters of SDAR services may use the additional and primary datastreams to target different types of receivers, as will be explainedbelow.

Legacy receiver: A current or existing SDAR receiver that is capable ofinterpreting first level data. Legacy receivers typically interpretsecond level data as noise.

Preamble: A known symbol or symbols in a transmission packet (typicallyused for synchronization).

Quadrature: A method of coding information that groups data bits andtransmits two separate signals on a carrier by summing the cosine andsine of the separate signals to produce a composite signal which may belater demodulated to recover both signals.

Second Generation Receiver: A SDAR receiver that contains hardwareand/or software enabling the receiver to interpret second level data(e.g., demodulator enhancements). Second generation receivers may alsointerpret first level data.

Second Level Data, Secondary Data and/or Hierarchical Data: Theadditional data that is superimposed on the first level data to create ahierarchically modulated data stream. Second level data may beinterpreted by SDAR receivers containing the appropriate hardware and/orsoftware to enable such interpretation (i.e., “second generation”receivers). Second level, or secondary, data may perform differentlyfrom first level, or primary, data.

Signal: A detectable physical quantity or impulse by which informationcan be transmitted.

Symbol: A unit of data (byte, floating point number, spoken word, etc.)that is treated independently.

Unitary Signal: A signal on a single channel or path.

Upconvert: To convert from a lower frequency signal (i.e., baseband) toa higher radio frequency signal for broadcasting.

Uplink: A communications channel or facility on earth for transmissionto a satellite, or the communications themselves.

Upmix: To combine multiple electrical signals to a radio frequencysignal for broadcasting.

Waveform: A representation of the shape of a wave that indicates itscharacteristics (frequency and amplitude).

QAM is one form of multilevel amplitude and phase modulation that isoften employed in digital data communication systems. Using atwo-dimensional symbol modulation composed of a quadrature (orthogonal)combination of two (2) pulse amplitude modulated signals, a QAM systemmodulates a source signal into an output waveform with varying amplitudeand phase. Data to be transmitted is mapped to a two-dimensional,four-quadrant signal space, or constellation. The QAM constellationemploys “I” and “Q” components to signify the in-phase and quadraturecomponents, respectively. The constellation also has a plurality ofphasor points, each of which represent a possible data transmissionlevel. Each phasor point is commonly called a “symbol,” represents bothI and Q components and defines a unique binary code. An increase in thenumber of phasor points within the QAM constellation permits a QAMsignal to carry more information.

Many existing systems utilize QPSK modulation systems. In such QPSKsystems, a synchronous data stream is modulated onto a carrier frequencybefore transmission over the satellite channel, and the carrier can havefour (4) phase states, e.g., 45 degrees, 135 degrees, 225 degrees or 315degrees. Thus, similar to QAM, QPSK employs quadrature modulation wherethe phasor points can be uniquely described using the I and Q axes. Incontrast to QAM, however, the pair of coordinate axes in QPSK can beassociated with a pair of quadrature carriers with a constant amplitude,thereby creating a four (4) level constellation, i.e., four (4) phasorpoints having a phase rotation of 90 degrees. Differential quadraturephase shift keying (D-QPSK) refers to the procedure of generating thetransmitted QPSK symbol by calculating the phase difference of thecurrent and the preceding QPSK symbol. Therefore, a non-coherentdetector can be used for D-QPSK because it does not require a referencein phase with the received carrier.

Hierarchical modulation, used in DVB-T systems as an alternative toconventional QPSK, 16-QAM and 64-QAM modulation methods, may better beexplained with reference to FIG. 1. FIG. 1 illustrates 64-QAMconstellation 100. Each permissible digital state is represented byphasors 110 in the I/Q plane. Since eight (8) by eight (8) differentstates are defined, sixty-four (64) possible values of six (6) bits maybe transmitted in 64-QAM constellation 100. FIG. 1 shows the assignmentof binary data values to the permissible states. In a 16-QAMconstellation, there are four (4) by four (4) different states and four(4) transmitted bits, in a 4-PSK constellation, there are two (2) by two(2) states and two (2) transmitted bits, and in a BPSK constellation,there is one (1) state and one (1) transmitted bit.

In systems employing hierarchical modulation schemes, the possiblestates are interpreted differently than in systems using conventionalmodulation techniques (e.g., QPSK, 16-QAM and 64-QAM). By treating thelocation of a state within its quadrant and the number of the quadrantin which the state is located as a priori information, two separate datastreams may be transmitted over a single transmission channel. While64-QAM constellation 100 is still being utilized to map the data to betransmitted, it may be interpreted as the combination of a 16-QAM and a4-PSK modulation. FIG. 1 shows how 64-QAM constellation 100, upon whichis mapped data transmitted at six (6) bits/symbol 116, may beinterpreted as including QPSK constellation 112 (which includes mappeddata transmitted at two (2) bits/symbol) combined with 16-QAMconstellation 114 (which includes mapped data transmitted at four (4)bits/symbol). The combined bit rates of QPSK and the 16-QAM data steamsis equal to the bit rate of the 64-QAM data stream.

In systems employing hierarchical modulation schemes, one (1) datastream is used as a secondary data stream while the other is used as aprimary data stream. The secondary data stream typically has a lowerdata rate than the primary stream. Again referring to FIG. 1, using thishierarchical modulation scheme, the two (2) most significant bits 118may be used to transmit the secondary data to second generationreceivers while the remaining four (4) bits 119 may be used to code theprimary data for transmission to the legacy receivers.

The present invention contemplates the use of hierarchical modulation ina SDAR system, while maintaining backward compatibility for legacyreceivers. Shown in FIG. 2 is a diagrammatic view of a SDAR system inwhich a hierarchical modulation scheme is employed. SDAR system 210includes first and second communication satellites 212, 214, whichtransmit line-of-sight signals to SDAR receivers 216, 217 located on theearth's surface. A third satellite may be included in other SDARsystems. Satellites 212, 214, as indicated above, may provide forspatial, frequency and time diversity. As shown, receiver 216 is aportable receiver such as a handheld radio or wireless device. Receiver217 is a mobile receiver for use in vehicle 215. SDAR receivers 216, 217may also be stationary receivers for use in a home, office or othernon-mobile environment.

SDAR system 210 further includes a plurality of terrestrial repeaters218, 219. Terrestrial repeaters 218, 219 receive and retransmit thesatellite signals to facilitate reliable reception in geographic areaswhere the satellite signals are obscured from the view of receivers 216,217 by obstructions such as buildings, mountains, canyons, hills,tunnels, etc. The signals transmitted by satellites 212, 214 andterrestrial repeaters 218, 219 are received by receivers 216, 217, whicheither combine or select one of the signals as receiver's 216, 217output.

FIG. 3 illustrates a block diagram of a SDAR communication system inwhich hierarchical modulation is utilized. In an exemplary embodiment ofthe present invention, SDAR communication system 300 includes SDARtransmitter 310, SDAR receiver 340 and terrestrial repeater 350. As inconventional SDAR communication systems, SDAR communication system 300will input data content 302, 304 and perform processing and frequencytranslation within transmitter 310. The digital data is transmitted overtransmission channel 330 to receiver 340 or terrestrial repeater 350.Generally, receiver 340 performs the converse operations of transmitter310 to recover data 302, 304. Repeater 350 generally re-transmits data302, 304 to receiver 340. Unlike conventional SDAR communicationsystems, however, transmitter 310, receiver 340 and repeater 350 of thepresent invention provide hardware enabling SDAR communication system300 to utilize a hierarchical modulation scheme to transmit and receivemore digital data than conventional systems.

SDAR transmitter 310 includes encoders 312, 322. The audio, video, orother form of digital content to be transmitted comprises primary inputsignal 302 and secondary input signal 304, which are typically arrangedas series of k-bit symbols. Primary input signal 302 contains primary,or first level, data and secondary input signal 304 contains secondary,or second level, data. Encoders 312, 322 encode the k bits of eachsymbol as well as blocks of the k-bit symbols. In other embodiments ofthe present invention, separate encoders may be used to encode theblocks of k-bit symbols, for example, outer and inner encoders. In anexemplary embodiment of the present invention, encoder 312 may encodeprimary data stream 302 using a block or a convolutional forward errorcorrection (FEC) algorithm, and encoder 322 may encode secondary datastream 304 using a turbo coding algorithm or a low density parity checkFEC algorithm. It is contemplated that other FEC encoding methods may beutilized to encode primary and secondary data streams 302, 204,including, for example, Hamming codes, cyclic codes and Reed-Solomon(RS) codes.

Again referring to FIG. 3, inner interleaver 316 multiplexes encodedsecondary content data stream 304 with encoded primary content datastream 302 to form a transmit data stream. This transmit data stream ispassed to mapper 317, which maps the data stream into symbols composedof I and Q signals. Mapper 317 may be implemented as a look-up tablewhere sets of bits from the transmit signal are translated into I and Qcomponents representing constellation points or symbols. FIG. 6 isrepresentative of an exemplary embodiment of the present invention, inwhich a hierarchical modulation scheme is employed and the constellationpoints are in accordance with either a uniform or non-uniform 8-PSKconstellation 600, where each phasor is represented by a three (3) bitsymbol composed of I and Q signals.

FIG. 4 shows QPSK constellation 400 for primary data having two (2)transmitted bits/symbol. Phasors “00”, “10”, “11”, “01” correlate to aphase of 45 degrees, a phase of 135 degrees, a phase of 225 degrees anda phase of 315 degrees, respectively. FIG. 5 shows BPSK constellation500 for secondary data having one (1) transmitted bit/symbol. Phasors“0” and “1” correlate to a phase of zero (0) and 180 degrees,respectively. When a secondary data symbol is added onto a primary datasymbol, constellation 600 of FIG. 6 is illustrative of the resultinghierarchical modulation.

Constellation 600 may be perceived as two (2) sets of superimposedmodulations—QPSK constellation 400 transmitting two (2) bits/symbol 620combined with BPSK constellation 500 comprising one (1) bit/symbol. Thefirst modulation is the primary QPSK data, which is represented by “x”marks 620, 622, 624, 626. In order to superimpose the secondary dataonto the primary data, the primary QPSK data is phase offset by theadditional, secondary data, which is represented by any of data points601, 602, 603, 604, 605, 606, 607, 608 depending on the phase offset.Positive phase offsets include phasors 602, 604, 606 and 608, andnegative phase offsets include 601, 603, 605 and 607.

Shown in FIG. 6, phase offset 610 is the offset angle relative to theQPSK symbol. As explained above, a typical QPSK constellation contains45 degree, 135 degree, 225 degree and 315 degree points. Thehierarchical data is represented by a phase offset relative to thosefour (4) degree points, and the phase offsets with the four (4) degreepoints represent a hierarchical (8-PSK) constellation. A uniform 8-PSKconstellation is created when offset angle 610 is 22.5 degrees. Everyother offset angle 610 creates a non-uniform 8-PSK constellation. Forexample, as shown in FIG. 6, a 15 degree phase offset relative toprimary data phasors 620, 622, 624, 626 produces a phase offsetcorrelative to phasors 601 (“000”) or 602 (“001”), 603 (“101”) or 604(“100”), 605 (“110”) or 606 (“111”), and 607 (“101”) or 608 (“010”),respectively. Gray coding is a method which may be used to make the bitassignments for the hierarchical constellation. For example, referenceis made to the secondary data bit (b2). Instead of making b2=0 anegative offset and b2=1 a positive outset, the hierarchicalconstellation may be configured so as to increase the bit error rate(BER) performance (e.g., b2=1 can be made a negative offset).

The amount of the phase offset is equal to the amount of power in thesecondary signal. The amount of energy in the secondary signal may notbe equal to the amount of energy in the primary signal. As phase offset610 is increased, the energy in the secondary data signal is alsoincreased. The performance degradation to the primary data signal isminimized by the perceived coding gain improvement as phase offset 610is increased. The application of the hierarchical phase modulation ontop of an existing QPSK signal containing primary data causes phaseoffset 610 to adjust either positively or negatively relative to thehierarchical data.

In general, a secondary data bit causes either a larger Q magnitude andsmaller I magnitude or a larger I magnitude and smaller Q magnitude.With FEC techniques utilized in encoders 312, 322, the I and Q signalsare used in conjunction with each other over a block of data. Thesetechniques give the appearance that the primary data bits are spreadover time, enabling the secondary data to appear somewhat orthogonal tothe primary data bits. Indeed, it has been shown in simulations that thesecondary data's impact on the primary data is somewhat orthogonal. Forexample, for a twenty (20) degree phase offset for secondary data, theprimary data has a one (1) decibel (dB) degradation when using a rate{fraction (1/3)} convolutional code with a constraint length of seven(7), followed by a (255, 223) RS block code (8 bits/symbol). However,when the primary data has no FEC coding, the impact of the twenty (20)degree phase offset is 4.1 dB. This data demonstrates a perceived codingimprovement of 3.1 dB in the case where phase offset 610 is set totwenty (20) degrees.

Again referring to FIG. 3, the FEC coding technique implemented byencoders 312, 322 spreads the primary and secondary data over many QPSKsymbols, which essentially spreads the energy over time and the I and Qbits. To overcome the unequal signal-to-noise ratio (“Eb/No”) betweenprimary data bits and secondary data bits, the amount of phase offset610 may be increased until the performance of the primary data is equalto the performance of the secondary data. However, as phase offset 610is increased, legacy receivers may have a difficult tine acquiring andtracking the desired primary data signal. By spreading the second levelbits over multiple symbols, spread spectrum coding techniques may beused to increase the amount of energy in the secondary bits. This allowsphase offset 610 to be adjusted and made more compatible with legacyreceivers. Additionally, the use of second level data spreading reducesoverall second level data throughput. Overall, several techniques may beutilized to maximize the performance of the secondary data. Thesetechniques include: increasing phase offset 610 to maximize thesecondary data energy per symbol; using multiple symbols per secondarydata bit; using more complex FEC algorithms, and using a beam steeringantenna to improve the performance of the secondary data (e.g., a highergain directional antenna for stationary reception and apointing/steering antenna for mobile reception).

Referring back to FIG. 3, after mapper 317 translates encoded andinterleaved primary and secondary data streams 302, 304, respectively,into I and Q components, the I and Q components are modulated bymodulator 318. Modulation enables both primary data stream 302 andsecondary data stream 304 to be transmitted as a single transmissionsignal via antenna 326 over single transmission channel 330. Primarydata stream 302 is modulated with secondary data stream 304 using one ofa number of modulation techniques, including including amplitude orphase and may include differential or coherent modulation (for example,BPSK, QPSK, differential Q-PSK (D-QPSK) or pi/4 differential QPSK (pi/4D-QPSK)). According to the technique that modulator 318 employs,modulator 318 may be any of amplitude or phase modulator. Eachmodulation technique is a different way of transmitting the data acrosschannel 330. The data bits are grouped into pairs, and each pair isrepresented by a symbol, which is then transmitted across channel 330after the carrier is modulated.

An increase in the capacity of the transmitted signal would not causebackwards compatibility problems with legacy receivers as long as thelegacy receivers may interpret the first level data. Second generationreceivers, however, are capable of interpreting both first and secondlevel data. Techniques may be employed to minimize the degradation inthe legacy receiver, including decreasing phase offset 610 to limit theamount of the second level data energy per symbol, limiting the amountof time over which the second level data is transmitted, and making thesecond level data energy appear as phase noise to the legacy receiver.

Referring back to FIG. 2, after modulator 318 modulates first datastream 302 and second level data stream 304 (FIG. 3) to create atransmission signal, transmitter 213 uplinks the transmission signal tocommunication satellites 212, 214. Satellites 212, 214, having a “bentpipe” design, receive the transmitted hierarchically modulated signal,performs frequency translation on the signal, and re-transmits, orbroadcasts, the signal to either one or more of plurality of terrestrialrepeaters 218, 219, receivers 216, 217, or both.

As shown in FIG. 3, terrestrial repeater 350 includes terrestrialreceiving antenna 352, tuner 353, demodulator 354, de-interleaver 357,modulator 358 and frequency translator and amplifier 359. Demodulator354 is capable of down-converting the hierarchically modulateddownlinked signal to a time-division multiplexed bit stream, andde-interleaver 357 re-encodes the bit-stream in an orthogonal frequencydivision multiplexing (OFDM) format for terrestrial transmission. OFDMmodulation divides the bit stream between a large number of adjacentsubcarriers, each of which is modulated with a portion of the bit streamusing one of the M-PSK, differential M-PSK (D-MPSK) or differential pi/4M-PSK (pi/4 D-MPSK) modulation techniques. Accordingly, if ahierarchically modulated signal is transmitted to one or bothterrestrial repeaters 218, 219 (FIG. 2), terrestrial repeaters 218, 219receive the signal, decode the signal, re-encode the signal using OFDMmodulation and transmit the signal to one or more receivers 216, 217.Because the signal contains both the first and second level data, theterrestrial signal maintains second level data bit spreading overmultiple symbols.

Also shown in FIG. 3, SDAR receiver 340 contains hardware (e.g., achipset) and/or software to process any received hierarchicallymodulated signals as well. Receiver 340 includes one or more antennas342 for receiving signals transmitted from either communicationsatellites 212, 214, terrestrial repeaters 218, 219, or both (FIG. 2).Receiver 340 also includes tuner 343 to translate the received signalsto baseband. Separate tuners may be used to downmix the signals receivedfrom communication satellites 212, 214 and the signals received fromterrestrial repeaters 218, 219. It is also envisioned that one tuner maybe used to downmix both the signals transmitted from communicationsatellites 212, 214 and the signals transmitted from repeaters 218, 219.

Once the received signal is translated to baseband, the signal isdemodulated by demodulator 344 to produce the I and Q components.De-mapper 346 translates the I and Q components into encoded primary andsecondary data streams. These encoded bit streams, which wereinterleaved by interleaver 316, are recovered by de-interleaver 347 andpassed to decoder 348. Decoder 348 employs known bit and block decodingmethods to decode the primary and secondary bit streams to produce theoriginal input signals containing the primary and secondary data 302,304. In other embodiments of the present invention, multiple decodersmay be used, e.g., outer and inner decoders. Receiver 340 may also use afeed forward correction technique to improve its detection of thesecondary data. By knowing the relative I/Q quadrant, receiver 340 maybe enhanced to perform better by having such a priori knowledge, whichassists in the detection of the transmitted signal. For example,referring to FIG. 6, if it is known from a priori first level dataknowledge that symbol 602 or 601 was transmitted at some point in time,and the received symbol lands at 604, it can be inferred by minimumdistance that the received second level data bit is a weak one (1) byutilizing feed forward correction. However, without feed forwardcorrection the second level data bit would have been detected as astrong zero (0). Therefore, feed forward detection utilizes the decodedsymbol with the detected offset (either positive or negative) todetermine the secondary data bit.

In another embodiment of the present invention, a method of enablingextra data bits from a hierarchical modulation scheme to be used totransmit additional data for each channel in a SDAR system iscontemplated. A flow chart illustrating this embodiment of the presentinvention as utilized in an SDAR communication system is shown in FIG.7. It is contemplated that the inventive method would be carried out bya receiver adapted to be used in a SDAR system. The receiver mayconcurrently process the receipt of first data stream 710 and seconddata stream 730. If first data stream 710 is valid as determined byerror checking at step 712, first data stream 710 is passed to a channeldata select at step 750. If first data stream 710 is selected and seconddata stream 730 is either independent or not valid, only first datastream 710 is decoded at step 720 at its original rate, e.g.,forty-eight (48) kbps. The decoded data from first data stream 710 isthen passed to an output unit at step 724.

If second data stream 730 is valid as determined by error checking atstep 732, then second data stream 730 is passed to the channel dataselect at step 750. If second data stream 730 is selected and isindependent from first data stream 710, only second data stream 730 isdecoded at step 740 at its original rate, e.g., sixteen (16) kbps. Thedecoded data from second data stream 730 is then passed to an outputunit at step 744.

If the receiver determines at step 712 that first data stream 710 isvalid and at step 732 that second data stream 730 is valid, both datastreams are passed to the channel data select at step 750. The channeldata select determines if second data stream 730 is an enhancement tofirst data stream 710. Audio enhancements may include audio qualityenhancements, audio coding enhancements such as 5.1 audio (i.e., aDolby® AC-3 digital audio coding technology in which 5.1 audio channels[left, center, right, left surround, right surround and alimited-bandwidth subwoofer channel] are encoded on a bit-rate reduceddata stream), data/text additions, album pictures, etc. If second datastream 730 is an enhancement to first data stream 710, the channel dataselect combines the two (2) data streams such that the combined signalhas a data rate greater than the first data stream's 710 data rate,e.g., 64 kbps. Thus, the sixteen (16) kbps data rate of second datastream 730 acts to increase the rate of first data stream 710 fromforty-eight (48) kbps to sixty-four (64) kbps. Combined data stream 758is then decoded at step 752 and passed to an output unit at step 756. Inan exemplary embodiment, when switching from first data stream 710 tocombined data stream 758, the increase in data rate is blended so as notto enable a quick change between first data stream 710 and combined datastream 758. If second data stream 730 is determined to be invalid, thechannel data select switches to a “first data level” only implementationand sends first data stream 710 to be decoded at step 720. The data rateof first data stream 710 remains at its original forty-eight (48) kbps.In an exemplary embodiment of this inventive method, a decrease in datarate is blended so as not to enable a quick change between first datastream 710 and combined data stream 758. Assuming that second datastream 730 becomes or remains valid, the receiver decodes combined datastream 758 at step 752 and provides combined data stream 758 to anoutput unit at step 756.

A further enhancement to the present invention involves superimposingthe secondary data by spreading it over a predetermined pattern and thusdoes not pose as many problems for legacy receivers. Hierarchicalmodulation data streams which have secondary data superimposed inpredictable patterns may be compensated for in the error correctionprocedures, e.g. FFC, of legacy receivers. This may cause a legacyreceiver to incorrectly adjust detection of the primary data and resultin a incorrect decoding of the primary data. By using a less predictablebut known pattern, a predetermined pattern or sequence, the hierarchicalmodulation scheme may optimize the secondary data transfer rate whileminimally effecting the performance of legacy receivers. For example,using a pseudo-random distribution of strong secondary data signals(i.e., a large phase offset) spread over the primary data, a secondgeneration receiver may optimize its performance with an a prioriknowledge of the secondary data distribution. Alternatively, wheremultiple secondary data streams are present, a direct sequence spreadspectrum (DSSS) approach may be used to have a fixed offset superimposedon each DSSS sequence (a Hadamard matrix is an effective DSSSimplementation). With either approach, the pattern is predetermined inthe sense that either the pattern itself or the method by which thepattern may be calculated is known.

As an example of an a priori system, an a prior pattern or sequence ofphase shifts may comprise ±(5°, 5°, 5°, 5, 10°, 10°, 10°, 15°, 15°, 20°,15°, 15°, 10°, 10°, 10°, 5°, 5°, 5°, and 5°). This offset sequence maybe represented by 1, 1, 1, 1, 2, 2, 2, 3, 3, 4, 3, 3, 2, 2, 2, 1, 1, 1,and 1 where the sequence is implemented by multiplying by ±5. Thus, eachprimary symbol has a variable amount of secondary data, to which asecond generation receiver may have software with this predeterminedsequence in its memory so that the secondary data may be detected andproperly decoded. However, a bell shaped pattern with little randomnesssuch as this example may pose problems for legacy receivers, where thelegacy receiver carrier and timing recovery algorithms are affected bythis predictable pattern such that it has causes additional carrier andtiming errors. Such errors may result in the primary data being decodedincorrectly. It is desirable to have the sequence appear random to thelegacy receiver.

An example which mitigates the effects on legacy receivers may have amore pseudo-random distribution, for example this second offsetsequence: 2, 1, 2, 2, 1, 3, 1, 3, 1, 4, 1, 3, 1, 3, 1, 2, 2, 1, and 2.While this second offset sequence has the same number and amounts ofoffsets, the pseudo-random distribution potentially mitigates the amountof carrier and timing recovery errors that a legacy receiver is likelyto use. Thus, the second generation receiver perceives both offsetsequences as a priori superimpositions on the primary data and hassoftware for correctly detecting both the primary and the secondarydata. However, a legacy receiver is less likely to make unwarrantedcarrier and timing adjustments on the pseudo-random second offsetsequence compared to the first offset sequence example. In fact, legacyreceiver performance may be further enhanced by using zero offsets forsome of the sequence values.

Another example of a pseudo-random sequence involves a transmissionusing DSSS techniques. In this example, multiple spread spectrumsecondary data is simultaneously superimposed across multiple primarydata symbols where the combination of spread spectrum signals at anygiven primary data symbol creates a random offset. In this example, eachspread spectrum signal is a binary sequence of fixed offsets that whenused for each secondary data creates a unique pseudo-random offsetsequence. The unique pseudo-random sequence is the sequence of thePseudo-Noise (PN) generator used in the DSSS modulation technique. Thesecondary data is superimposed on the primary data symbol beingtransmitted and is spread over the PN sequence. With this example, thesecondary data can have the same amount of energy per symbol, and isthus less susceptible to transmission errors. One type of sequence thatprovides advantages is a Hadamard matrix sequence in the DSSS technique,which provides performance enhancements, as each element of the sequenceis orthogonal to the other elements thus making detection andcomputation of the secondary data relatively easy from a computationalperspective.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

1. A method for transmitting at least two levels of data in ahierarchical transmission system comprising the steps of: generating afirst modulated signal based on first input data; superimposing at leasta second modulation on the first modulated signal based on at least asecond input data, the second modulation being spread across a pluralityof symbols in the first modulated signal in a predetermined pattern tocreate a modified signal; transmitting the modified signal; decoding themodified signal by performing a first demodulation of the firstmodulated signal then a second demodulation to obtain the first inputdata and the second input data.
 2. The method of claim 1 wherein thesuperimposing step uses a plurality of offset sequence values as thepredetermined pattern to spread the at least second modulation acrossthe first modulated signal.
 3. The method of claim 2 wherein theplurality of offset sequence values includes a pseudo-randomdistribution of offset sequence values.
 4. The method of claim 2 whereinthe plurality of offset sequence values includes at least one zerooffset value.
 5. The method of claim 2 wherein the at least a secondmodulated signal involves direct sequence spread spectrum modulation andthe predetermined pattern is a pseudo-noise distribution.
 6. The methodof claim 5 wherein the predetermined pattern includes a Hadamard matrixsequence.
 7. A receiver for receiving at least two levels of data in ahierarchical transmission system comprising: an antenna for receiving RFsignals; a demodulator coupled to said antenna for downconvertingreceived RF signals; a first detector coupled to said demodulator, saidfirst detector having a first output and capable of providing digitalinformation based on a first level of data on said first output; and atleast a second detector coupled to said demodulator, said at least asecond detector having at least a second output and capable of providingdigital information based on at least a second level of data on said atleast a second output, said at least a second detector being adapted todetect the at least a second level of data as a predetermined patternspread over the first level of data.
 8. The receiver of claim 7 whereinsaid at least a second detector uses a plurality of predetermined offsetsequence values as the predetermined pattern to detect the at least asecond level of data.
 9. The receiver of claim 8 wherein the pluralityof offset sequence values includes a pseudo-random distribution ofoffset sequence values.
 10. The receiver of claim 8 wherein theplurality of offset sequence values includes at least one zero offsetvalue.
 11. The receiver of claim 8 wherein said at least a seconddetector is capable of detecting a direct sequence spread spectrummodulation with the predetermined pattern including a pseudo-noisedistribution.
 12. The receiver of claim 111 wherein said at least asecond detector includes a predetermined pattern comprising a Hadamardmatrix sequence.
 13. A transmitter for transmitting at least two levelsof data in a hierarchical transmission system comprising: an encoderadapted to receive a first and at least a second input data, saidencoder capable of providing digital information based on a first levelof data on an output stream, said encoder capable of providing digitalinformation based on at least a second level of data on said outputstream, said encoder being adapted to superimpose the at least a secondlevel of data as a predetermined pattern spread over the first level ofdata; a modulator coupled to said encoder for upconverting said outputstream; and an antenna coupled to said modulator for transmitting RFsignals based on the upconverted output stream.
 14. The transmitter ofclaim 13 wherein said encoder uses a plurality of predetermined offsetsequence values as the predetermined pattern to superimpose the at leasta second level of data over the first level of data.
 15. The transmitterof claim 14 wherein the plurality of offset sequence values includes apseudo-random distribution of offset sequence values.
 16. Thetransmitter of claim 14 wherein the plurality of offset sequence valuesincludes at least one zero offset value.
 17. The transmitter of claim 14wherein said modulator is capable of modulating said output stream as adirect sequence spread spectrum modulation and said predeterminedpattern includes a pseudo-noise distribution.
 18. The transmitter ofclaim 17 wherein said predetermined pattern comprises a Hadamard matrixsequence.